The present invention relates generally to liquid crystal displays (LCDs), and more particularly to active matrix liquid crystal displays (AMLCDs) having gray scale with an improved viewing angle.
AMLCDs are devices well known in the art for their utility in visually displaying data and images in a variety of applications such as in aviation cockpits. However, a significant problem with AMLCDs has been the difficulty in achieving gray scale with adequate viewing angle. This is due to the fact that the brightness versus drive voltage (BV) curves for AMLCDs vary significantly as a function of viewing angle.
One way to achieve gray scale in an AMLCD is to drive individual pixel elements at a number of discrete drive voltages to achieve discrete output intensities or brightness values. Pixel elements in AMLCDs do not actually generate light, but rather, act as light valves in the sense that the transmittance of a particular pixel element changes as the corresponding drive voltage is increased or decreased. For purposes of this application, pixel element "output intensities" or "brightness values" are phrases intended to mean the apparent intensity of a pixel element resulting from backlighting and the pixel element's transmittance. Also for purposes of this application, it should be understood that references to a pixel element having a particular color are actually referring to the pixel element's apparent color resulting from an illuminating output intensity and the presence of a color filter associated with the pixel element. Finally, for purposes of this application a "pixel" is defined to be a physical region on the display which includes one red pixel element, one green pixel element, and one blue pixel element in close proximity to one another and generally being controlled at least somewhat independently of one another. The combination of output intensities of the three different colored pixel elements in each pixel are optically blended by the eye of the viewer to create the appearance that the pixel has a single color and intensity. Each of the pixel elements may in turn be divided into separate pixel sub-elements having the same color and generally occupying the same physical space as the pixel element.
Generally, the discrete drive voltages will include a threshold voltage V.sub.T below which the particular individual pixel element is not illuminated (has no output intensity), a saturation voltage V.sub.S at and above which the maximum output intensity for the pixel element is substantially achieved, and a number of discrete voltage levels between V.sub.T and V.sub.S. Each of the discrete voltage levels between V.sub.T and V.sub.S corresponds to a particular output intensity, for the pixel element, between the non-illuminated state and the maximum output intensity.
In AMLCDs, the non-illuminated and the maximum output intensity states are relatively viewing angle independent when compared to the intermediate output intensity levels. However, the intermediate drive voltages result in output intensities which are heavily dependent on viewing angle. The result is poor gray scale performance of the AMLCD at wide viewing angles.
One method of improving gray scale performance in AMLCDs is disclosed in U.S. Pat. No. 4,840,460 to Bernot et al, which is assigned to Honeywell Inc. The Bernot et al patent describes a method of providing half-tone gray scale over wide viewing angles in which each pixel element of the display is subdivided into a plurality of pixel sub-elements all having the same color. Each of the pixel sub-elements, which has an effective capacitance, is connected in series with a separate control capacitor. Each pixel sub-element/control capacitor combination is connected in parallel with the other pixel sub-element/control capacitor combinations to form a single pixel element. In the disclosed preferred embodiments, the capacitance characteristics of the various pixel sub-elements and control capacitors are chosen so that a different drive voltage is necessary to "turn on" each pixel sub-element.
As one pixel sub-element is about to enter an optical saturation state in response to an increasing drive voltage, the next pixel sub-element is near its threshold of optical activity. As the drive voltage is increased, the number of pixel sub-elements in the saturation state increases, but no more than one pixel sub-element is driven between the threshold and saturation states at any one time. As a result, no more than one pixel sub-element will have appreciable angular dependence at any one time, and the average gray scale performance of the pixel element as a whole will be largely viewing angle independent.
While methods such as the one disclosed in Bernot et al can be used to produce an AMLCD having improved gray scale performance over wider viewing angles, they do have several disadvantages. For example, these techniques increase total drive voltage requirements and have a negative impact on yield since adjacent elements in the LCD matrix must have different properties. The maximum drive voltage applied to the first pixel sub-element of each pixel will have to be considerably higher than V.sub.S in order to achieve optical saturation in the other pixel sub-elements